In radially-pulsating cool giants, outward moving material collides against
the gas lifted during the previous cycle and now falling back inward. The
resulting shock is hot enough to excite emission in the Balmer lines of
hydrogen. Such shocks develop deep within the atmosphere, interior to the
outer layers where absorption by TiO molecules occurs. The absorption by
TiO begins around 4200 Å and, by superposition of successive bands (they
all decline toward the red), their combined absorption grows rapidly
stronger with wavelength (Fluks et al. 1994). As a consequence, emission
in H\(\delta \)  (4101 Å) and higher Balmer terms exits the atmosphere
unscathed by TiO, and their flux declines with increasing upper \(n\) -quantum
number in the usual way. What happens to the red of H\(\delta \) ? Going from
H\(\gamma \)  to H\(\beta \)  and then H\(\alpha \) , we move into deeper and deeper
overlapping absorption by TiO bands and the observed emission in the
line becomes more and more absorbed. Saying it a different way
[1]}, [2]}, in pulsating M giants the
strongest line is H\(\delta \)  and the intensity of successive Balmer emission
lines declines either going to the blue or to the red (see exemplary
spectrum for LQ Sgr presented by [3]}). In most
astrophysical environments, the observed emission in H\(\alpha \)  is invariably
stronger than in H\(\beta \) , while in Miras the opposite is the case
[4]}, because of the TiO absorption in the outer
atmospheric layers. This offers a clear distinction between the emission
originating from the accretion disk in an acc-SySt (in terms of
equivalent widths: EW(H\(\alpha \) ) \(>\)  EW(H\(\beta \) )) from that expected to
come from the internal shock regions of an M-type radial pulsator
(EW(H\(\alpha \) ) \(\le \)  EW(H\(\beta \) )).
